This invention pertains to wound capacitors and, more particularly, to such capacitors of the metallized film type. The particular problem to which this invention is directed is to produce a pair of capacitors having one plate in common which have improved performance and are easier to manufacture.
My U.S. Pat. No. 3,921,041 discloses and claims a dual capacitor and method of making it. Dual capacitors made in accordance with that invention have been very successful. However, it would be desirable to improve them in at least two respects.
First, it would be desirable to improve their "life test" performance. During a life test the capacitors are subjected to extremes of temperature and voltage. For example, a 370 volt AC rated capacitor will be tested at 480 volts AC in an oven at 80.degree. C. If the capacitors are basically "good", the standard measure of performance is the change in capacitance value (.DELTA.C) after a test of 500 to possibly as much as 2000 hours duration. Usually, the test parameter allows for a loss of 3% from the initial value. In testing dual capacitors made in accordance with the above-referenced patent having small and large capacitance values, it has been found that the smaller capacitor suffers a greater negative loss (as much as -3%) whereas the larger may only lose 1%. One objective of this invention is to produce a dual capacitor which exhibits performance characteristics which more closely resemble a single capacitor.
As taught by the above-referenced patent, a barrier material is inserted into the cylindrical body to electrically separate the non-common plates of the two capacitors. The end of the body is then coated by an electric arc metal spraying technique. The metal spray is then removed from the barrier to prevent electrical "bridging." Thereafter, electrical leads are soldered to the end, one within and one without the barrier. It would be desirable to employ polycarbonate film for the barrier, rather than the polypropylene which is usually used. One reason is that polycarbonate has a "slicker" surface and the metal is more easily removed. Another reason is that, as larger capacitors are manufactured, larger electrical lead tabs must be soldered in place. The heat tends to penetrate the polypropylene barrier. The practical difficulty, however, is that the cost of polycarbonate is approximately ten times that of polypropylene. Accordingly, a second objective of this invention is to enable the use of polycarbonate without substantially increasing the cost of the dual capacitor.
The manner in which these objectives are achieved will be apparent from the following description and appended claims.